Panel switches have been widely used as switches employed in input operating sections of a variety of electronic devices. The panel switch is formed of a movable contact unit including a movable contact retained by a base sheet.
FIG. 9 shows a sectional view illustrating a part of a panel switch including a movable contact unit in which a conventional movable contact is used. FIG. 10 shows an exploded perspective view of the panel switch shown in FIG. 9. The conventional movable contact and the movable contact unit using the conventional movable contact are described hereinafter with reference to FIGS. 9 and 10.
In FIGS. 9 and 10, base sheet 101 made of insulating resin film covers the top faces of plural movable contacts 105. Each movable contact 105 is made of a resilient and electrically conductive thin metal plate, and has an external appearance of a circular dome bowing upward. Application of depressing force onto the center of the domed shape bows the domed shape downward, and removal of the depressing force restores the downward domed shape to its original shape due to the restoring force of the resilient metal plate. In other words, this conventional movable contact is an inside-out type movable contact.
At the center section of movable contact 105, hemispherical projections 107 protruding downward are formed at intervals of 120 degrees and equidistant from the center. Meanwhile an adhesive layer is formed on an underside of base sheet 101, and movable contact 105 is retained by the adhesive layer. This is generally called a movable contact unit.
Wired board 115 includes a fixed contact pair including outer contact 116 and center contact 117 opposing each one of plural movable contacts 105. Wired board 115 includes spacer member 102 on its top face, which has plural holes 103. Each one of movable contacts 105 is placed in respective holes 103. The top faces of respective movable contacts 105 are covered with base sheet 101.
Respective movable contacts 105 are placed corresponding to the place of each one of the pairs of outer contact 116 and center contact 117. An operating button (not shown) is placed corresponding to the place of movable contact 105, so that a panel switch is formed. This panel switch is mounted to an electronic device.
Each individual switch of the panel switch is structured this way: the lower end of an outer periphery of movable contact 105 is placed on outer contact 116, and the underside of the center section including hemispherical projections 107 confronts center contact 117 with a space in between. Base sheet 101, movable contact 105, and hemispherical projections 107 form movable contact unit 110.
The operation of the conventional panel switch discussed above is described hereinafter. An operating button (not shown) is depressed, thereby applying depressing force to the central section of corresponding movable contact 105 via base sheet 101. When the depressing force exceeds a given amount, movable contact 105 is turned over, and three hemispherical projections 107 are brought into contact with central contact 117. This mechanism allows outer contact 116 to become electrically conductive to corresponding central contact 117, so that the switch is turned on.
When the depressing force is removed, movable contact 105 restores to the original domed shape bowing upward due to its own restoring force. Then three hemispherical projections 107 leave central contact 117, so that outer contact becomes electrically open with respect to central contact 117.
The foregoing panel switch can be constructed with a light-weight and thin body, and can be operated by push-operation, so that they are widely used particularly in portable devices.
On the other hand, a single switch, e.g. as shown in FIG. 11, is also widely used in various electronic devices. The push-on switch shown in FIG. 11 comprises the following elements:                box-like housing 120 made of resin;        outer contact 121 and central contact 122 both placed on the inner bottom face of the recess of the opening;        movable contact 105 discussed above and placed on outer contact 121 at its outer periphery; and        operating unit 125 movable up and down and built-in on movable contact 105.        
The operation of this push-on switch is described hereinafter. Operating unit 125 is depressed for applying depressing force to the central section of movable contact 105, and when the depressing force exceeds a given amount, movable contact 105 turns over and three hemispherical projections 107 touch central contact 122 placed under projections 107. As a result, terminals respectively led out from outer contact 121 and central contact 122 become conductive to each other via movable contact 105. When the depressing force is removed from operating unit 125, movable contact 105 restores to its original shape and pushes up operating unit 125 to the original place, so that the terminals become electrically disconnected from each other again.
The prior art related to the present invention is, e.g. Unexamined Japanese Patent Publication No. 2000-322974.
When the switch formed of conventional movable contact 105 discussed above is used, the inside-out position of movable contact 105 causes hemispherical projections 107, which are placed at three places and formed independently of each other, to touch central contacts 117 or 122 placed under projections 107. However, the touching status is a point contact, and yet, hemispherical projections 107 touch central contacts 117 or 122 at generally determined points. When the switch has been frequently used, the counterpart points of central contacts 117 or 122 to projections 107 thus sometimes get dented.